Together with the recent trends towards smaller and lighter mobile communication terminals, there is also a demand for slimmer structures. Conversely with the continued demand for miniaturization of their size, there is also a demand for diversified functions in the mobile communication terminals.
In this manner, with the miniaturization and diversification of functions in mobile communication terminals, it is needed to minimize the space occupied by the antenna inside a mobile communication terminal, adding to the difficulty in designing the antenna.
In addition, there is a trend toward convergence terminals, wherein a single terminal can handle services for various frequency bands, and accordingly, broadband characteristics and multiband characteristics are becoming the main factors in antennas. For example, there is a demand for an antenna that can support multiband services including short-distance communication service such as Bluetooth, mobile communication services, and wireless LAN services.
Antennas generally used in mobile communication terminals include the helical antenna and the planar inverted F antenna (PIFA).
In the case of a helical antenna, which is designed to protrude outside of the terminal, it is difficult to design an aesthetic appearance and an external appearance suitable for portability; however, an internal structure for this has not been studied, and therefore, it is not appropriate for the current trend that demands an internal antenna.
An inverted F antenna is an antenna designed to have a low-profile structure that makes it possible for installing inside a terminal. Among the beams generated by the current induced in the radiating part in an inverted F antenna, the beams facing the ground are re-induced to attenuate the beams facing the human body, thereby providing improved SAR characteristics, while the beams induced in the direction of the radiating part are strengthened, thereby providing directivity. Moreover, the inverted F antenna operates as a rectangular micro-strip antenna, in which the length of the flat rectangular radiating part can be halved, and thus can implement a low-profile structure.
Such an inverted F antenna offers many advantages as regards miniaturization and radiating characteristics, and is the most widely used internal antenna at present, but has narrowband characteristics, thus presenting difficulty in designing for multiband and broadband characteristics.
In order to overcome these problems of an inverted F antenna, an internal antenna using electromagnetic coupling has been disclosed by the inventor in Korean patent application No. 2008-2266, and FIG. 1 is a drawing illustrating the structure of the internal antenna using electromagnetic coupling disclosed by the inventor.
The internal antenna using electromagnetic coupling according to the structure in FIG. 1 can obtain greater broadband characteristics than an inverted F antenna, but there are instances where the required broadband characteristics cannot be obtained in certain ground structures and terminal structures. Thus, there is a need for a structure that can complement this matter.